The present invention relates to an electric power steering apparatus for applying assist torque of a motor to a steering mechanism of a vehicle to assist a driver in a steering operation.
An electric power steering apparatus of this kind is described in Japanese Patent No. 4453012. The electric power steering apparatus in Japanese Patent No. 4453012 is provided with a first reference model for setting target steering torque on the basis of a steering angle, and a second reference model for setting a target turning angle of a steering system on the basis of steering torque. Driving of a motor is controlled on the basis of a control signal (assist command value) set by both reference models (ideal models). In such a case, torque feedback control is executed to cause actual steering torque to follow target steering torque. A first assist component obtained by the torque feedback control is then included in the assist command value, so that the steering torque can be always set at an optimal value. Moreover, turning angle feedback control is executed to cause an actual turning angle to follow a target turning angle. A second assist component obtained by the turning angle feedback control is then included in the assist command value, so that reverse input vibration from a steerable wheel can be cancelled.
However, with the electric power steering apparatus described in Japanese Patent No. 4453012, a deviation between the actual turning angle and the target turning angle sometimes increases remarkably in execution of the turning angle feedback control. In such a case, control becomes unstable. That is, when the deviation between the actual turning angle and the target turning angle is too large, it is impossible to apply assist torque corresponding to the deviation. When the turning angle feedback control is continued in such a situation, shortage of assist torque is promoted and an appropriate target turning angle cannot be calculated. Accordingly, control becomes unstable. In such a case, it is desirable to remove the second assist component from the assist command value in order to stabilize control. It is also desirable to continue calculation of the second assist component while the second assist command value is removed from the assist command value. In such a manner, a settling time for the turning angle feedback control can be shortened when an increase in the deviation between the actual turning angle and the target turning angle is reduced and the second assist component is included again in the assist command value.
On the other hand, when the second assist component is removed from the assist command value as described above, characteristics of a control system are changed substantially. Here, an ideal model is set in view of the convergence in the control system. However, when the characteristics of the control system are changed, it becomes difficult to ensure the convergence and self-excited vibration may occur in the ideal model. When self-excited vibration occurs in the ideal model, it is impossible to calculate an appropriate target turning angle. Thereby, when an increase in the deviation between the actual turning angle and the target turning angle is reduced and the second assist component is included again in the assist command value, a settling time for the turning angle feedback control becomes longer and assist torque becomes unstable. This leads to driver discomfort.